Deciphering deuterostome phylogeny: molecular, morphological, and palaeontological perspectives

Abstract

Deuterostomes are a monophyletic group of animals that include the vertebrates, invertebrate chordates, ambulacrarians and xenoturbellids. Fossil representatives from most major deuterostome groups, including some phylum-level crown groups, are found in the Lower Cambrian, suggesting that evolutionary divergence occurred in the Late Precambrian, in agreement with some molecular clock estimates. Molecular phylogenies, larval morphology and the adult heart/kidney complex all support echinoderms and hemichordates as a sister grouping (Ambulacraria). Xenoturbellids are a relatively newly discovered phylum of worm-like deuterostomes that lacks a fossil record, but molecular evidence suggests that these animals are a sister group to the Ambulacraria. Within the chordates, cephalochordates share large stretches of chromosomal synteny with the vertebrates, have a complete Hox complex and are sister group to the vertebrates based on ribosomal and mitochondrial gene evidence. In contrast, tunicates have a highly derived adult body plan and are sister group to the vertebrates based on the analyses of concatenated genomic sequences. Cephalochordates and hemichordates share gill slits and an acellular cartilage, suggesting that the ancestral deuterostome also shared these features. Gene network data suggest that the deuterostome ancestor had an anterior-posterior body axis specified by Hox and Wnt genes, a dorsoventral axis specified by a BMP/chordin gradient, and was bilaterally symmetrical with left-right asymmetry determined by expression of nodal.

Regeneration of amphioxus oral cirri and its skeletal rods: implications for the origin of the vertebrate skeleton

“…It is arguable whether the first cellular cartilage was acquired as a cirral skeleton, because oral cirri were only found in the extant amphioxus and no comparable structure has been found in ascidians or vertebrates, including agnatha. However, it is not unreasonable to assume that oral cirri were present in the ancestors of the chordates if they were likely to be swimming filter feeders (Gans, '89; Smith and Swalla, 2009), for which oral cirri would have been adaptive. Cirri may have been lost in the vertebrates and ascidians when the former became hunters and the latter turned sessile and acquired tunic instead.…”

Higher-level metazoan relationships: recent progress and remaining questions

“…The discovery of putative deuterostome fossils from the Cambrian, such as vetulicolians (Aldridge et al 2007;Shu et al 2001), yunnanozoans (Mallatt and Chen 2003;Shu et al 2003), vetulocystids (Shu et al 2004), and cambroemids (Caron et al 2010), has added to evidence provided by preradial stem group echinoderms (Smith 2005) that external gill slits are a primitive deuterostome character lost during the early evolution of echinoderms (see reviews by Smith and Swalla 2009). The deuterostome affinities of at least some of those groups (notably the vetulicolians and yunnanozoans) are disputed by other palaeontologists, who posit that possible moult assemblages indicate a relationship to Ecdysozoa (e.g.…”

Testing character-evolution models in phylogenetic paleobiology: a case study with Cambrian echinoderms

“…Despite their diminutive size, geological antiquity, and narrow paleogeographic and stratigraphic ranges, the significance of cinctans to understanding early echinoderm evolution, as well as their evolutionary implications surrounding ancestral character states in ancient deuterostomes (Smith and Swalla, 2009), has led to a substantial amount of interest to decipher their paleobiology. Recent advances in cinctan paleobiology include efforts to better understand patterns of taxonomic diversity (Zamora andÁlvaro, 2010), ontogeny and development (Smith, 2005;Zamora et al, 2013b), life mode and feeding ecology , convergence and adaptive evolution (Zamora and Smith, 2008) and phylogenetic relationships (Friedrich, 1993;Sdzuy, 1993;Smith and Zamora, 2009;Zamora et al, 2013b).…”